1. Field of the Invention
The present invention relates to a radiation cassette for storing therein a plate-like radiation image recording medium on which the radiation image information of a subject is recorded.
2. Description of the Related Art
There is known a system in which a stimulable phosphor is used to record radiation image information of a subject such as a human body or the like, and the recorded radiation image information is reproduced on a photographic photosensitive medium such as a photographic film, or output to a display unit such as a CRT.
The stimulable phosphor is a phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of such radiation, and, when subsequently exposed to applied stimulating light such as visible light, emits light in proportion to the intensity of the stored radiation energy. The stimulable phosphor is normally shaped as a plate and used in the form a stimulable phosphor plate.
It has been customary, when applying radiation, e.g., X-rays, to a subject such as a human body or the like, to record the radiation image information of the subject directly on a photographic film. The photographic film is then subjected to a development process to produce a visible image thereon, which will be used for medical diagnosis or the like.
A radiation image recording medium, such as a stimulable phosphor plate or a photographic film, is usually stored in a cassette, which is loaded into an imaging apparatus. In the imaging apparatus, X-rays are applied to the radiation image recording medium through the cassette. One known cassette for storing such a radiation image recording medium is disclosed in Japanese Laid-Open Patent Publication No. 11-271894, for example.
FIG. 20 of the accompanying drawings shows a cassette 1, as disclosed in the above Laid-Open Patent Publication, which comprises a pair of case members 2a, 2b fastened to each other by screws and having an opening 3 defined in one end thereof. The cassette 1 also includes a tray 4 removably disposed in the opening 3, and a radiation image conversion plate 5 fixed to the tray 4. The tray 4 has a cap 6 which closes the opening 3 when the tray 4 is fully inserted into the case members 2a, 2b. 
The tray 4 is relatively heavy as a whole because of the cap 6 mounted on the tray 4 and due to the radiation image conversion plate 5 fixed thereto. Therefore, the mechanism by which the tray 4 is inserted into and removed from a radiation image recording and reading apparatus loaded with the cassette 1 likewise must be considerably large and heavy.
Furthermore, since the tray 4 is inserted into a narrow opening 3 of the cassette 1, the tray 4 must be accurately controlled in order to properly insert the tray 4 into the opening 3. As a result, the mechanism for inserting and removing the tray 4 is required to be precise in structure and highly accurate in operation, making the radiation image recording and reading apparatus highly costly to manufacture.